More specifically, the object of the invention is a digital device that makes it possible to calculate, in real time, a set-point signal that is corrected and intended in particular to be used in units for the generation of magnetic field gradients.
The latter are commonly employed in spectrometry by nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) systems.
In a known manner, the gradients are produced by coils placed inside the magnet that generates the constant primary magnetic field. These coils are controlled by current amplifiers, where their set point usually is given in voltage and generally consists of a train of pulses.
This set point is generated by a digital device whose input is a sequence switch, constituting a programmable pulse generator, and the output is a digital-to-analog conversion stage that provides set-point voltage.
The pulse generator is programmed based on the type of analysis that is to be performed by the spectrometer or the imager.
FIG. 1, attached, shows the different essential constituent components of a system for the generation of gradients.
A common configuration of such a system makes it possible to generate field gradients along the three axes (X, Y and Z, the latter being collinear with the primary field), as shown in FIG. 2, attached.
Other configurations of systems are also known and employed.
It is also known that, during the variation of the magnetic field in a gradient coil, Foucault currents appear on conductive elements that are outside of the coil and are subject to this field. These currents produce a magnetic field that opposes the field of the gradients.
By taking into consideration, for example, a gradient coil through which a current pulse passes, the build-up time of the magnetic field created by this coil will be influenced by the field that is produced by the Foucault currents. This phenomenon is illustrated in FIG. 3, attached (illustration of the effect of Foucault currents on the magnetic field that is generated by a gradient coil).
The degradation of the gradient field by the one produced by the Foucault currents has a detrimental influence on the performances of the spectrometer or the imager. It is consequently necessary to reduce to the maximum the effect of the Foucault currents on the gradient field.
For this purpose, a correction (of the pre-accentuation type, for example) can be applied to the voltage set point so as to compensate for the effect of the opposite field (FIG. 4, attached, illustrates such a correction of the effects of the Foucault currents).
The digital solutions that are proposed as of this time for making such a correction are primarily of two types: namely the entirely hardware solutions (based on stateful machines, for example), and software solutions (employed on a DSP [digital signal processing] signal-processing microprocessor, for example).
The first type of solution results in a rigid and fixed implementation, and the second type of solution experiences limited performance.
The document U.S. Pat. No. 5,349,296 discloses an NMR imager that integrates in particular a system for supplying gradient coils, able to provide set-point signals specific to each channel and formed from series of codes describing the wave profiles to be applied.
According to this document, the supply system can also integrate, without their nature being specified in advance, means for pre-emphasis correction acting on the analog signals that are delivered to different channels and using predefined parameters.
Thus, these correction means do not act on digital data, and they do not make it possible to make a correction by recursive operation.
The result is a limited, totally fixed correction method whose performance cannot be easily estimated in advance and is in any case limited.